1. Field of the Invention
The present invention relates to a multilayer electrophotographic photoreceptor, and to an image forming method, an image forming apparatus and a process cartridge for an image forming apparatus using the photoreceptor.
2. Discussion of the Background
Electrophotographic methods such as the Carlson process and its various modified processes are widely used for copiers, printers, etc. As an electrophotographic photoreceptor (hereinafter referred to as a photoreceptor) for use in the electrophotographic method, an organic photoreceptor has been typically used because of its low cost, the ease of mass producing it, and because it is non-polluting. A mechanism of forming an electrostatic latent image on a photoreceptor include:
(1) irradiating a photoreceptor after it has been charged;
(2) the irradiated light is then absorbed in a charge generation material of the photoreceptor;
(3) the light-absorbed charge generation material generates a charge carrier;
(4) the charge carrier is injected into a charge transport material and transported through the charge transport layer (or the photosensitive layer) along an electric field generated by the charge; and
(5) the charge carrier neutralizes the charge on the surface of the photoreceptor to form an electrostatic latent image.
Specific examples of organic photoreceptors include the following photosensitive layers:
(1) photoconductive resin layers typified by polyvinylcarbazole (PVK);
(2) charge transfer complex type photosensitive layers typified by 2,4,7-trinitrofluorenon (PVK-TNF);
(3) pigment dispersion type photosensitive layers typified by phthalocyanine-binder; and
(4) functionally-separated photosensitive layers using a combination of a charge generation material and a charge transport material.
Among these photoreceptors, the functionally-separated photoreceptors have attracted attention and come into practical use.
Various organic photoreceptors have been developed. However, in order to make a good photoreceptor requires good electrophotographic properties such as high sensitivity, high potential, high potential retainability, good potential stability, low residual potential and proper spectral properties; good mechanical durability such as high abrasion resistance; good chemical stability against heat, light and discharge-induced products (e.g. ozone and Nox), etc.
Above all, a need exists for a photoreceptor having a small diameter because there is a demand for downsizing electrophotographic systems. Therefore, there is a demand for a photoreceptor having good durability against abrasion increasing in proportion to the number of copies.
Thus, mechanical durability, which typically means abrasion resistance, is in strong demand. However, the conventional organic photoreceptors and the electrophotographic processes using these photoreceptors do not have high durability due to the low abrasion resistance of the organic materials used. In addition, the demand for a photoreceptor having good abrasion resistance has increased because the thickness of a photosensitive layer has to be decreased so as to produce higher definition images.
The reason why the thickness of a photosensitive layer has a particularly big influence on producing higher definition images is considered as follows.
Among positive and negative carriers formed in the charge generation layer of a negatively chargeable multilayer organic photoreceptor by light irradiation, the negative carrier (an electron) is absorbed in the substrate and the positive carrier (a hole) is transported through the charge transport layer to the surface of the photoreceptor to be combined with an electron thereat, resulting in disappearance of the hole.
Due to the disappearance, the electric field to attract the hole gradually decreases, and the hole moves toward a non-lighted area.
This is called a carrier scattering phenomenon in a direction of the surface of the photoreceptor, and because of it formation of a latent image faithful to the irradiated light is prevented. This results in formation of a poor image having low resolution.
The thickness of the charge transport layer has a big influence on the carrier scattering phenomenon, and making the thickness smaller is very effective relative to maintaining good image resolution.
In addition, laser irradiation is different from irradiation by a halogen lamp, etc. because the incident photon current speed of a laser is about 107 times as fast as that of a halogen lamp. Therefore, the density of the formed carriers is extremely high, and the electric field strength of the charge generation layer decreases because charges flow into the charge transport layer. Thus, the carrier transport speed is influenced, resulting in late arrival of the carrier formed by irradiation of the center portion of the laser beam to the surface of the photoreceptor. The thus formed space charge distribution tends to cause carrier scattering in a direction parallel to the surface of the photoreceptor, resulting in deterioration of image resolution.
Japanese Laid-Open Patent Publication No. 57-30846 discloses a method for improving abrasion resistance of an organic photoreceptor, which includes a protective layer including a filler formed from a metal or a metal oxide. The object of this method is to increase the transparency of the protective layer and prevent an increase of residual potential by using a filler having an average particle diameter of not greater than 0.3 xcexcm. In addition, Japanese Laid-Open Patent Publication No. 4-281461 discloses a method, in which a charge transport material is included in a protective layer together with a filler to prevent an increase of residual potential while maintaining good abrasion resistance. Japanese Laid-Open Patent Publications Nos. 53-133444 and 55-157748 disclose a method, in which an organic acid is included in a protective layer together with a filler, and Japanese Laid-Open Patent Publication No. 2-4275 discloses a method, in which an electron accepting material is included in a protective layer, to prevent an increase of residual potential. Japanese Laid-Open Patent Publication No. 8-234455 discloses a method, in which the difference in refractive index between a filler in a charge transport layer and the charge transport layer is not less than 0.1, and the layer includes 1xc3x97104 to 2xc3x97105 pieces of filler particles having a particle diameter of from 1 to 3 xcexcm per 1 xcexcm2.
Further, Japanese Laid-Open Patent Publication No. 8-339092 discloses a method, in which a surface layer of a photoreceptor includes inorganic particles having a volume average particle diameter of from 1 xcexcm to 2 xcexcm and a ratio of the number average particle diameter to the weight average particle diameter of from 1 to 2.
However, although the photoreceptors produced by these methods have good abrasion resistance, these photoreceptors have the following drawbacks:
(1) toner film tends to be formed thereon;
(2) the surface thereof is poorly cleaned because contact of a cleaning blade with the photoreceptors deteriorates due to agglomeration of a filler in the protective layer which is caused by poor dispersion of the filler; and
(3) the electrostatic stability and durability of a the photoreceptor deteriorate.
Therefore, the desired photoreceptor properties have not been achieved yet. Particle conditions of inorganic particles included in a protective layer have a big influence on the above-mentioned properties of the resultant photoreceptor. This is partly disclosed in Japanese Laid-Open Patent Publication No. 8-339092. However, the actual particle conditions of the inorganic particles in the protective layer are not described therein. Instead, it is only described that the particle diameter distribution in a dispersion liquid is sharp and large particles are not included therein. Therefore, a photoreceptor having good electrophotographic properties and good durability has not been developed yet.
Accordingly, an object of the present invention is to provide an electrophotographic photoreceptor which can be used for a high-durability and high-speed digital electrophotographic image forming apparatus using a laser as a writing light source, and which can produce a high resolution image by the laser irradiation.
Another object of the present invention is to provide a photoreceptor which has high mechanical durability while having a thin layer to produce a high-resolution image.
Yet another object of the present invention is to provide an image forming method, an image forming apparatus and a process cartridge for an image forming apparatus using the photoreceptor, by which images having good image qualities can be produced for a long period of time without producing an abnormal image such as image tailing which is produced by a conventional high-abrasion-resistance photoreceptor.
A further object of the present invention is to provide a photoreceptor having a surface layer which is abraded in a very small amount to remove surface-deteriorating substances adhered on the surface layer, and to provide an image forming apparatus and method including a cleaner cleaning the deteriorating substances. It cannot be said that a photoreceptor having the highest possible abrasion resistance is always good. It is, instead, preferable that the surface of a photoreceptor is abraded in a very small amount by removing surface-deteriorating substances to keep the surface of the photoreceptor clean.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a photoreceptor including at least an electroconductive substrate; and a photosensitive layer overlying the electroconductive substrate and including at least a charge generation layer; a charge transport layer; and a protective layer including a filler, wherein the filler is dispersed in the protective layer so as to occupy an area of from 3 to 5% of a cross section of the protective layer; the filler has a particle diameter distribution having a peak at a diameter of from 0.2 to 0.3 xcexcm; and the particles of the filler having a particle diameter of not less than 0.3 xcexcm occupies an area of from 10 to 30% of the area of all the filler in the cross section of the protective layer.
In another aspect of the present invention, an image forming method using the photoreceptor is provided.
In yet another aspect of the present invention, an image forming apparatus using the photoreceptor is provided.
In a further aspect of the present invention, a process cartridge using the photoreceptor is provided.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.